1. Field of the Invention
The present invention relates to real-time position sensing of a moving part within a cryogenic refrigeration system. More specifically, the present invention relates to the use of position sensing of moving parts in a cryogenic compressor or cryogenic refrigeration system using precision sensors and support circuitry capable of accurately determining the location of a moving part.
2. Description of the Background Art
Cryogenic refrigeration systems or “cryocoolers,” are becoming increasingly commonplace and are used in a variety of applications where very cold temperatures are necessary or desired. These applications include cooling of sensor electronics or optics in military and commercial equipment used in satellites and space flight, superconducting electronics and in research and development.
Typical cryogenic refrigeration systems have an ability cool from 70-80K in a single stage system and in the range of 2.3K in advanced, multistage systems. Closed system cryogenic refrigeration systems include the Stirling, Gifford-McMahon J-T and pulse tube head types which cool by alternating the compression and expansion of a working fluid, commonly, helium. The Stirling and Gifford-McMahon systems incorporate a mechanical, reciprocating displacer to move the helium to the system regenerator and to remove heat and return the gas to a system compressor. The high-pressure helium is expanded in the displacer module connected to a regenerator and cold head. The pulse tube system does not include a mechanical displacer but rather cyclically compresses and expands the helium as it moves through the heat exchanger by use of a rotary valve to generate an oscillating compression-expansion cycle.
Common to each of the Stirling, Gifford/McMahon, J-T and pulse tube systems is a compressor assembly in which the expanded helium or other working fluid that is returned from the system is compressed by use of a piston. The compressor assembly includes one or more pistons in a compression volume or cylinder for the compression of the returned helium. These cryogenic refrigeration systems commonly use a linearly oscillating piston driven by an electric “voice coil” motors where the piston assembly is supported by a flexure bearing. The piston oscillation is typically controlled by an active position control servo operating on position sensor feedback. It has been determined that active position control improves thermodynamic efficiency of the unit, helps reduce vibration, and prevents a piston from impacting end stops during operation. It has also been determined that control of fluid flow and movement of the expansion chamber (displacer) must be continuously and accurately timed.
In order to have better control over the temperature of a cold head in a cryocooler system, there is a need to receive continuous indication of the cryopump piston, the position of the piston in a displacer as well as the position of balancing weights in vibration dampened systems.
Optical Encoder Technology
Current position control designs for moving components in a cryocooler system use linear variable differential transformers (LVDT) as position sensors. This method works well, but the special design and low volume production associated with incorporating an LVDT into an existing cryocooler system is expensive and results in lengthy design delays. Further, new cryocooler designs require a new or redesigned LVDT systems to allow the pistons to operate over a variable stroke range or to fit a different space allocation. This undesirably lengthens the design cycle and results in relatively high non-recurring engineering costs. Accordingly, there exists a need for a less expensive, precision position sensor system that can readily be adapted to existing cryocoolers or to future cryocooler designs.
Optical encoder technology is well developed, well known and widely used in other applications. Encoder components are low-cost, off-the-shelf items with the exception of the user-specified, printed pattern on the encoder's index plate. This pattern is usually made per user specification and there are many vendors for such devices with proven and cost effective practices.
An optically encoded position sensor of the type described herein simplifies manufacturing and reduces cost relative to LVDT technology for a number of reasons. Existing encoder units use several readily available, commercial off-the-shelf or easily manufactured components, are easy to install and align in a cryocooler, are compact, lightweight, and are highly reliable. These elements lend themselves to low-cost production and incorporation into existing cryocooler technology.
As is well known in the art, optical encoders are devices that have two major sub assemblies. One subassembly is an index plate, which is connected to a moving component, is a system requiring position detection. The index plate is connected to and moves with the specified component. The index plate generally comprises a transparent plate with one or more longitudinal sections with different markings on each of them. The second subassembly, referred to herein as an emitter/detector or transmit/receive unit, is mounted on a stationary member on or within the compressor housing and typically consists of a light source (LED or another) and a light detector. When the encoder is assembled into the compressor, the light source and detector are positioned in a way that there is a gap between them whereby the detector picks up the light from the light source. The index plate occupies this gap (without contacting either of the parts) and when it moves, the markings block the light and as a result, signals are generated by the detector's circuitry.
There are two types of encoders preferred for use in the claimed invention, each of which is well know in industry. The first is an incremental encoder containing a single section on the index plate with multiple demarcations of a known distance between. The output of this type of encoder is the movement speed (number of line counts per time unit) or the linear displacement relative to a known previous position.
The second major type of encoder is the absolute position encoder. In this type of encoder, the index plate consists of multiple sections, each section of which contains multiple demarcations so as to generate a digital word for every discrete position with a length equivalent to the number of sections (2 sections=2 bits etc.). The number of light source-detector sets and also the number of signal lines in this absolute encoder type is equal to the number of sections on the index plate. The output of this type of encoder is the absolute position of the encoder's plate relative to an absolute zero established during the system's assembly or calibration. The position is expressed as a digital word and the length of this word determines the system's precision. For example, a 10-bit word yields 1024 discrete positions. The precision in this case equals full stroke length divided by 1024.
Both types of encoders described above as transmissive devices, with a transparent index plate and opaque demarcations, but can be reflective index plates with the light source and detector positioned on the same side of a reflective index plate.
An alternative preferred embodiment incorporates an absolute-incremental encoder that requires only four I/O pins passing through the pressure housing for each encoder.
The use of absolute-incremental encoder is the proposed method for replacing the LVDT in cryocoolers with an optical encoder using a limited number of I/O pins in the hermetically sealed compressor unit.
The absolute-incremental encoder index plate consists of three sections on the same physical piece. One section has a number of markings with a known spacing between them that match the precision needed from the system (in the same way as in an incremental encoder). The other two sections have similar markings arranged in pairs, where the lines in each pair (one line in each section) have a smaller gap between them than the gap between the pairs. These two sections are used for direction of movement detection. During operation, receiving the signals from the two light detectors corresponding to these two sections in one order indicate one direction of movement and vice versa. The bigger gap between the pairs indicates to the system that a new pair is being read. Receiving a signal from the same detector twice without receiving a signal from the other detector indicate to the system a reversal in the direction of movement
An “absolute position reset line” is marked across all three sections. The system resets to a known absolute position each time all three detectors transmit a simultaneous signal (all three signals received within a predetermined time frame). The actual reset takes into account the position of only one detector (the one that was used during the calibration process) to avoid ambiguity. The reset event is not required every cycle but because the system's accuracy improves with the number of resets, the absolute position reset line is located in the middle of the stroke.
The absolute-incremental system uses the direction of movement indication to either add or subtract the position increment to/from the absolute position obtained in the reset process. The side from the absolute position is also taken into account.
When the precision required by the system cannot be met by one incremental encoder section because the line width and spacing exceed the printing capabilities, more sections may be added with their markings shifted from one another to form a finer uniform division of the full motion stroke. Each such section adds one light source and detector. The signals from these additional sets are wired into the same signal line eliminating the need for additional signal lines and I/O pins.
The number of I/O pins required by the proposed system is three for each encoder, two for the direction detection and one for the incremental output. In addition, two I/O pins supply power to all the encoders in one cryocooler.
A single encoder design can be, rapidly adapted to work in different cooler designs by simply changing one component: an easily manufactured light-modulating grid. This passive element modulates light intensity seen at a detector such that motion steps can be accurately counted. The remaining elements of the sensor (light source, detectors, and electronics) are unaffected by changes in piston motion over the range of Stirling-class cooler designs planned for development over the next five to ten years.